Hot mix asphalt (hereinafter “HMA”) or stone matrix asphalt (hereinafter “SMA”) is commonly used in highway construction for paving both in new road construction and in maintenance of existing roads which have surface cracks or potholes. Similarly, HMA and SMA mixes are used in airfield construction for paving both new runways and in maintenance of existing runways. These HMA and SMA mixes are currently produced in stationary plants utilizing liquid asphalt stored at temperature above the melting point typically above 230° to 250° F. This liquid asphalt is then pumped into a gradation of heated stone aggregated which has a specific particle size distribution properly suited for the paving or repair application. Different gradations are used for different applications, usually ranging in particle size from material passing 200 mesh (0.003 inch) to 0.75 or 1 inch diameter. About 5% to 7% by weight asphalt is mixed with the aggregate to produce HMA or SMA. Plastic asphalt utilizing polymers as binders for the aggregate have also been used.
The creation and use of HMA and SMA mixes can be challenging, since both the HMA and SMA mixes and the asphalt used in the HMA and SMA mixes require the use of large amounts of energy to be maintained at relatively high temperatures until used. For example, the transport of such asphalt to be used as binder in the various asphaltic mixture applications can be problematic. Typically, asphalt is handled in bulk form due to the fact that the material tends to stick together even after separation when stored in the same container. During processing in anticipation for use, the asphalt must be kept heated at over 275° F. in liquid form for subsequent handling and mixing. For example, asphalt binder is handled as a hot liquid from its origination point, usually the refinery, with heated rail cars or trucks. The asphalt binder remains heated in its liquefied state through shipment to and use in the asphalt mix plant. Since such material must be maintained at elevated temperatures for transfer to storage tanks and any transfer from one container to another until ultimate use, significant amounts of energy in the form of heat must be expended in order to maintain the asphalt in its liquefied state. It is estimated that, in the United States alone, over 60 billion pounds of asphalt must be kept continuously heated. Depending on any additives or other materials added to the asphalt, maintaining the asphalt at elevated temperatures for prolonged periods can adversely affect the properties of the asphalt.
Similarly, problems arise in the transport of the HMA and SMA mixes. To pave roads which may be 30 or 50 miles or more from the mix plant, very careful and difficult coordination is required between the production of the hot HMA and SMA mixes at the plant, the use of large trucks with consequent traffic control, the need to maintain the HMA and SMA mixes at a controlled temperature to the jobsite, and the coordination of the paving crew and equipment for proper installation of the HMA and SMA mixes for paving and patching roads, bridge decks, parking lots, airport runways, and other paving jobs. Proper temperature of the HMA and SMA mixes when applied is important and difficult to control due to hauling long distance and often presents problems.
Situations which present particular difficulty using these fixed plant trucking systems are maintenance, patching, and off-season work. For example, remote airfields may become damaged thus rendering them unusable. Also roads may be damaged in remote locations creating situations involving high security and safety risk where quick local action is required. Often maintenance or pothole patching may only require a few tons of HMA or SMA at a remote location. To send a truck with three to five tons of HMA from a mix plant may take several hours of expensive labor and equipment and often excess material is dumped on vacant land, creating an environmental hazard to be resolved later. The conventional system is expensive and inefficient for use for small amounts of maintenance, patching, and off-season work, especially in remote areas.
One alternative to hot mix patching is an emulsified asphalt mix applied at ambient temperature with solvents and water evaporation after placement. These “cold” mix systems are inferior in quality and often must be replaced. Also, they can be very expensive.
Another alternative uses small, portable mix units, typically 5 to 15 tons/hr, to heat recycled asphalt (hereinafter “RAP”) for patching. These units do not produce HMA or SMA of adequate quality to meet pavement grade specifications. These units normally use open flame to heat the mix which has previously been coated with binder. This open flame can degrade the RAP mix which already has the binder coating. In order to reheat the RAP mix, the aggregate which has already been coated with asphalt or polymer binders requires a large amount of heat to get the aggregate to a working temperature of 300° F. to 350° F. Such a temperature will burn or thermally degrade the asphalt or polymer coating creating a substandard product which will not meet the standards or specifications of most state departments of transportation or specifications for new hot mix. Also, large amounts of smoke and unhealthy gases and smells are produced.
Another method used for winter repair is the “hot box” manufactured by companies like RayTek of Charlestown, N.H. These devices use indirect heat, such as gas fired plates, which conduct the heat in a chamber to reheat stock piles of previously coated aggregate. The advantage of these devices is they do not use open direct flame which would burn and degrade the binder coated aggregate. However, they keep the coated aggregate at high temperatures sometimes for days while they are transported to job sites for small patching jobs. Such long periods of high temperatures also degrade the asphalt binder to a substandard product. Further, hot mix used in these devices requires continuous heating using much energy and only supply limited quantities of mix. If the total amount is not used, reheating further degrades the mix.
Thus, a need exists for an on site hot mix system that can solve such difficult problems and facilitate quick repair with materials that can be stored on site at ambient temperature.